Method and vessel for treating an underwater bed

ABSTRACT

In a method for treating the bed of a body of water by means of a directed water jet, which water jet is moved over the bed ( 5 ) in accordance with an elongated path, the water is pumped under pressure on board a vessel ( 1 ) and then discharged, via a pressure line ( 2 ) connected to said vessel ( 1 ), a small distance above, and directed towards the bed ( 5 ). With this procedure, a trench ( 6 ) can be formed in the bed.

[0001] The invention relates to a method for treating the bed of a body of water by means of a directed water jet, which water jet is moved over the bed in accordance with an elongated path.

[0002] A method of this type is disclosed in WO-A-92/18701. In this known method a pumping installation is used which contains a pump screw which has a relatively large diameter. Using this pumping installation, a relatively large amount of water can be discharged at relatively low speed, in such a way that a trench is made in the bed when the pumping installation is dragged over the bed.

[0003] The pumping installation is provided with positioning means with which it has to be held at a certain distance above the bed. By means of tow cables, it is moved forwards by a vessel, for example an ocean-going tug, which also contains the power source for driving the pump screw.

[0004] The known method and installation have the disadvantage that the dredging process is difficult to control. The pumping installation must be held under water in the correct position with respect to the bed, which can have associated complications.

[0005] The aim of the invention is, therefore, to provide a method of the type described above which does not have these disadvantages. This is achieved in that water in pumped under pressure on board a vessel and is then discharged, via a pressure line connected to said vessel, a small distance above, and directed towards, the bed.

[0006] With the method according to the invention, virtually any equipment, in particular that for generating the water jet, can be installed on board the vessel itself. This means that that part of the installation which remains underwater can remain relatively simple, which is beneficial for the reliability and also results in lower costs.

[0007] The water can be pumped at an overpressure between 0.01 bar and 20 bar, preferably at an overpressure between 0.01 bar and 8 bar. Depending on the distance of the spray nozzle of the pressure line to the bed to be treated, various results can be obtained by means of such pressures. In any case however, the distance of the nozzle to the bed is equal to maximally 10 times the diameter of the water jet defined by the nozzle or the envelope of several jets defined by the nozzle. Preferably, said distance to the bed is equal to 6 times the diameter of the water jet defined by the nozzle or of the envlope of several jets defined by the

[0008] In case a relatively large distance of the nozzle to the bed is selected, the phenomenon of entrainment of surrounding water by the water jet occurs. Thereby, a water flow having a lower speed is generated, but the flow rate and the transverse dimension of said water flow have become greater.

[0009] Over a distance of about 6 times the diameter of the water jet defined by the nozzle, neither the impulse nor the energy of the water flow thus generated diminishes significantly. Such water flow is very suitable for jetting a trench in the bed. The flow is deflected away transversely, leading to transport of bed material and to deposing of such material at both sides of the shallow trench thus formed.

[0010] Thus, according to a first possibility, it is possible to form a trench in the bed of the body of water using the method according to the invention. In this case, the pressure and the flow rate of the water jet must be chosen sufficiently high that the bed material is completely fluidised and sprayed away.

[0011] In case the distance towards the bed is smaller, no such wide water flow can occur. As a result, the water jet delivered by the nozzle hits the bed in concentrated shape, leading to penetration of water into the bed. Thereby, the coherence of the soil mixture is broken, and “dilution” occurs. In this process, water is fed into the particle skeleton, as a result of which the bed material material obtaines liquid-like properties.

[0012] According to this possibility, the water in fed into the bed material in such a way that the particle skeleton thereof partially loses its cohesion and/or strength (dilution). In this case, complete fluidisation does not occur and the bed material retains a certain cohesion and/or strength. This variant of the method according to the invention can be used when burying a cable or pipeline laid in or on the bed. In this case, the pressure and the flow rate of the water jet are so chosen that the cohesion and/or strength of the particle skeleton of the bed material is reduced down to such a depth that the pipeline or cable sinks into the bed under the influence of its own weight.

[0013] Depending on the bed material, a cable or pipeline to be buried can also be laid sometime after dilution of the bed material. The interval between the treatment of the bed material and laying of a cable or pipeline depends on the consolidation time of the bed material. In the case of another similar variant, it is possible to remove the buried article from the bed with the exertion of little force by diluting the bed material to an appropriate degree in the vicinity of the bottom of, for example, a cable, pipeline or other buried article.

[0014] Good results can be obtained if the flow rate of the jet is 0.25 to 20.0. in particular 0.25 to 5.0, cubic metres per second.

[0015] The invention also relates to a vessel for carrying out the method described above. A vessel of this type has a pressure line which has a spray nozzle, as well as means for holding the pressure line against the force of reaction of the water jet discharged from the spray nozzle.

[0016] By means of a pressure line of this type it is possible to discharge a water jet under a considerable impulse, in such a way that a broad and deep trench can be formed in the bed by spraying.

[0017] In order to be able to hold the spray nozzle in the correct position, ballast means can also be provided. As an alternative, however, it is also possible to provide dynamic positioning means for holding the pressure line in position.

[0018] Various types of vessel can be chosen for the vessel to be used with the method. These vessels can be specially developed vessels; advantageously, however, a hopper suction dredger can be used, comprising a well in addition to at least one line which can be played out down to the bed of a body of water, pressure means being provided which are connected to the end of the line which is connected to the vessel.

[0019] In the case of the method according to the invention, the hopper suction dredger is not used to exert a suction effect but to exert a spray effect.

[0020] As is known, a hopper suction dredger of this type comprises suction means for drawing up bed material under suction via the line. According to the invention, switching means are now provided for connecting the pressure means or the suction means to the line, as desired.

[0021] Using a hopper suction dredger of this design, it is possible both to use the method according to the invention and to carry out the normal suction operation.

[0022] The nozzle can have a diameter 0.1 to 5.0 metres, preferably 0.2 to 1.0 metre, in size.

[0023] In connection with the burying of a pipeline or cable which has already been laid, the end of the nozzle can terminate in a multipiicity of smaller nozzles, which are of rectangular shape and the individual cross-sectional surface area of which is from 0.005 to 1.0 m² in size. The small nozzles can also have a circular shape, in which case the diameter of the individual nozzles is from 0.1 to 1.0 metre in size. Using a nozzle of this type, the water jet can be metered easily over, for example, the circumference of the pipeline or cable.

[0024] An example of a vessel for carrying out the method according to the invention will be described in sore detail below with reference to the figures.

[0025]FIG. 1 shows a side view of a vessel of this type.

[0026]FIG. 2 shows a front view.

[0027] The vessel, which is indicated in its entirety by 1, is constructed as a hopper suction dredger, which is provided with a line 2. A nozzle 3 is fitted at the end of the line 2,: the line 2 itself can be paid out in an known manner, by means of cables 4 and 4′. to just above the bed 5 of a body of water.

[0028] Pumping means for pumping water via the line 2 to nozzle 3 are installed on board the hopper suction dredger in a manner which is not shown in sore detail.

[0029] As the vessel 1 sails, a trench 6 is flushed out in the bed by the discharge of a water jet from the nozzle 3. The bed material which is sprayed away partly collects in the banking 7 alongside the trench 6. With this procedure, the bottom of the trench can be diluted. The area of the bed material which is diluted is indicated by 8 in FIG. 2. The cohesion and/or strength is partly retained, but has become so low that a pipeline or cable which has already been laid is able to sink into the bed.

[0030] Ballast means or dynamic positioning means (which incidentally are not shown), can be provided in the vicinity of the nozzle 3 to hold the line 2 in place against the force of reaction of the water jet 3. 

1. Method for treating the bed of a body of water by means of a directed water jet, which water jet is over the bed in accordance with an elongated path, characterised in that water is pumped under pressure on board a vessel and is then discharged, via a pressure line connected to said vessel, a small distance above, and directed towards, the bed.
 2. Method according claim 1, wherein the water is pumped at an overpressure between 0.01 bar and 20 bar.
 3. Method according claim 1 or 2, wherein the water is pumped at an overpressure between 0.1 bar and 8 bar.
 4. Method according to one of claims 1-3, wherein the water emerges under overpressure from a nozzle the distance of which to the bed is equal to maximally 10 times the diameter of the water jet defined by the nozzle or of the envelope several water jets defined by the nozzle.
 5. Method according to one of the preceding claims, wherein the water emerges under overpressure from a nozzle the distance of which to the bed is equal to maximally 6 times the diameter of the water jet defined by the nozzle or of the envelope of several water jets defined by the nozzle.
 6. Method according to one of claims 1-5, wherein a trench is formed in the bed.
 7. Method accord to one of claims 1-6, wherein the water is fed into the bed material such a way that the particle skeleton thereof partially loses its cohesion and/or strength (dilution).
 8. Method according to claim 7, for burying a pipeline or cable, wherein the pressure and the flow rate of the water jet are so chosen that the cohesion and/or strength of the particle skeleton of the bed material is reduced down to such a depth that the pipeline or cable sinks into the bed under the influence of its own weight.
 9. Method according to claim 7, for removal of, for example, a pipeline, cable or other article, wherein the pressure and the flow rate of the water jet are so chosen that the cohesion and/or strength of the particle skeleton of the bed material is reduced to an extent such that the articles can be removed from the bed material under a small force, which is of the order of magnitude of the weight of the article.
 10. Method according one of claims 1-9, wherein the flow rate of the jet is 0.25 to 20.0 cubic metres per second.
 11. Vessel for car out the method according to claim 1, wherein a pressure line is provided which has a spray nozzle, as well as means for holding the press line against the force of reaction of the water jet discharged from the spray nozzle.
 12. Vessel according to claim 11, wherein ballast means are provided for holding the pressure line in place.
 13. Vessel according to claim 12, wherein dynamic positioning means are provided for holding the pressure line in position.
 14. Vessel according to claim 11, 12 or 13, constructed as a hopper suction dredger comprising a well in addition to at least one line which can be played out down to be of a body of water, wherein pressure means are provided which connected to the end of the line connected to the vessel.
 15. Vessel according claim 143 comprising suction means for drawing up bed material under suction via the line, wherein switching means are provided for connecting the pressure means or the auction means to the line, as desired.
 16. Vessel according to claim 11, 12, 13, 140 or 151, wherein the nozzle has a diameter of 0.1 to 5.0 metres in size.
 17. Vessel according to claim 16, wherein the nozzle has a diameter of 0.2 to 3.0 m in size.
 18. Vessel according to claim 16 or 17, wherein the end of the nozzle terminates in a multiplicity of smaller tubular nozzles, the diameter of the individual nozzles being from 0.1 to 1.0 m in size.
 19. Vessel according to one of claims 11-18, wherein the nozzle has a rectangular shape with an internal cross-sectional surface area of from 0.005 to 20 m²
 2. 20. Vessel according to claim 19, wherein the end of the nozzle terminates in a multiplicity of smaller individual nozzles, which have a rectangular shape, the cross-sectional surface area of the individual nozzles being from 0.005 to 1.0 m².
 21. Vessel according to one of claims 11-20, wherein the end of the nozzle or of the individual nozzles is/are flexible and/or capable of moving flexibly. 